As we move towards the realization of optical routing in fiber networks, it is becoming increasingly important to provide signal-processing functions such as switching, in optical form at the network nodes. Optical switching is expected to become more and more important as wavelength division multiplexing reaches further into networks, greatly expanding the number of optical paths available. By using integrated optical components to perform the network node routing functions, advantages in terms of functionality, size, speed, and efficiency are achieved.
The integrated optical multimode interference (MMI) coupler has been the subject of much attention and research in recent years see, for example: L. B. Soldano, et al. in a paper entitled "Planar Monomode Optical Couplers Based on Multimode Interference Effects," J. Lightwave Technol., vol. 10, no. 12, pp. 1843-1849, 1992; M. Bachmann, et al. in a paper entitled "General self-imaging properties in N.times.N multimode interference couplers including phase relations," Appl. Opt., vol. 33, no. 18, pp. 3905-3911, 1994; and L. B. Soldano et al., in a paper entitled "Optical multi-mode interference devices based on self-imaging: principles and applications," J. Lightwave Technol., vol. 13, no. 4, pp. 615-627, April 1995. All references in this document are herein incorporated by reference. It has been shown that MMI couplers can be used in generalized Mach-Zehnder interferometer configurations, to actively route and switch optical signals as detailed by: L. B. Soldano et al., in a paper entitled "Optical multi-mode interference devices based on self-imaging: principles and applications," J. Lightwave Technol., vol. 13, no. 4, pp. 615-627, April 1995; and R. M. Jenkins, et al., in a paper entitled "Novel 1.times.N and N.times.N integrated optical switches using self-imaging multimode GaAs/AlGaAs waveguides," Appl. Phys. Lett., vol. 64, no. 6, pp. 684-686, February 1994.
Generally, the 1.times.N switch is controlled by applying N sets of phase shifts to N arms. For small N, the phase shifts are quantized into a small number of discrete levels, and control is possible using digital logic circuitry to drive the phase shifters. As N increases, however, the driving conditions of the switch become more complex. The number of discrete phase shift levels, the number of independent phase shifts, and the maximum phase shift required all increase. As a result, the operation of larger switches requires elaborate control circuitry and higher power levels.
It would be advantageous to provide an apparatus that improves the control of the 1.times.N switch.